Feature Highlight: Todos

Introduced the 'Todos' feature, which aggregates notifications for assigned issues, merge requests, and @mentions into a single list. This feature reuses the existing Activity feed system and leverages Rails' ActiveModel::Dirty API to track attribute changes on issues and merge requests. It automatically marks todos as 'Done' when the user takes action on the associated issue or merge request, or when the issue/merge request is closed/merged by the user. The implementation also includes logic to handle changes in associations like labels and assignee_id.